Kid.
Every time he called her that, he was pushing her away. From the very start, he believed that saying it would keep her at arms length. First time, he followed it with saying something that he knew was a complete lie. 'First sign of trouble, you're just going to cut and run'.
Who was he trying to convince? (Definitely not zipper head.) She ran into a group of armed men with a pathetic stick planning to do the gods know what. She put herself in danger, with no thought of herself, all to save absolute strangers.
That was the first time he felt anything but the constant numbness all those years in the Tin Suit, when that slip of a girl ran to save his family, brandishing that stick like a sword. All these years, she was the only one to help.
So he pushed her away. Called her kiddo, hoping that he would not have to admit that he saw her as the brave woman she was. Or at the very least, let her or anyone else see the glimmer of admiration for her in his eyes.
Because once he admitted that she was the cause for the beginning cracks in the tin that covered his heart, he was a goner.